Several types of closures or fasteners are available for reclosing a previously opened package. For example, it is common to use mechanical reclosable fasteners, such as slider zippers, clips, tabs, interlocking strips, and the like. These mechanical closures can be bulky, complex structures that require separate molding and fabrication steps prior to being joined to the package. Further, film rolls and other packaging materials incorporating such fasteners can be unwieldy and difficult to handle due to the added bulk from the fasteners. Additionally, such fasteners can add significant material and production costs to a package. Mechanical fasteners may also not form an airtight seal upon closure. For instance, when in a closed position, slider zippers can have an undesirable small air channel or gap due to bridging of interlocking flanges between an end-stop and the slider. These mechanical fasteners can be applied in form, fill, and seal operations, but such a process can require complex manufacturing steps to apply, interconnect, and align the features of each structure. For these reasons, mechanical reclosable fasteners can add undue complexity, cost, and expense into the manufacture of such packages.
Adhesive-based reclosable fasteners provide one alternative to the mechanical fasteners discussed above. Adhesive-based fasteners, however, present other challenges in both the manufacture and formation thereof. For example, thermoplastic elastomers (TPE), which are sometimes called thermoplastic rubber, have been used to form reclosable pressure sensitive adhesive (PSA) type fasteners. Some types of TPE copolymers (especially certain styrenic block polymers) demonstrate high cohesive properties, and at the same time have low tack levels or a reduced tendency to adhere to unlike materials. TPEs can be effective at forming reclosable fasteners; however, such TPEs tend to have undesirably high cohesive properties that render them difficult for use as a reclosable fastener in packaging applications because the TPE can delaminate from the package substrate rather than peel at the cohesive interface. Additionally, the TPEs may need to be dissolved in a solvent so that the TPE can be printed in a transverse or machine web direction as an intermittent pattern onto the package substrate. In some cases, organic solvents suitable for use as a carrier with TPEs may not be acceptable for contact with food items. Pattern application of TPEs in the form of an aqueous dispersion has been contemplated; however, creating TPE dispersions in water that are suitable for flexographic or rotogravure printing processes is technically difficult to accomplish. Even if a technically feasible method for dispersing TPE in water were to be discovered, the high cost of removing the water carrier after the coating is applied may render pattern coating of aqueous TPE dispersions commercially infeasible for high volume consumer packaging applications.
Other types of pressure-sensitive adhesives (PSAs) may be useful as reclosable fasteners for packages; however, common PSA reclosable fasteners generally have high tack levels. Tack is a property of an adhesive material that generally enables the material to form a bond with the surface of another material upon brief or light pressure. Tack is often considered as a quick stick, an initial adhesion, or a quick grab characteristic of a material. Ordinary PSA adhesives generally cannot be surface-printed onto materials that are intended to be run on commercial package forming equipment. Typical problems encountered when attempting to run PSA surface coated materials on packaging equipment include: blocking where the material does not unwind freely from a roll due to unacceptable back-side adhesion; picking where there is undesirable and unintended transfer of adhesive material to equipment surfaces, such as rollers, mandrels and filling tubes; poor tracking, such as the inability of the material to stay in proper alignment as it passes through the packaging machine; and jamming where the material is unable to slide over equipment surfaces and binds up.
PSA reclosable fasteners may also present concerns to a consumer using a formed package. If the package is used to contain a crumbly (i.e., a cookie, cracker, and the like) or shredded product (i.e., shredded cheese and the like), the high tack of most PSAs may cause the crumbs or shreds to stick to the PSA, which reduces the effectiveness of the adhesive to form a sufficient closure due to contamination of the PSA surface from the food product. A PSA fastener that is sufficiently contaminated with product will generally not form an adequate closure seal because the crumbs that adhered to the PSA generally do not allow the PSA to adhere to the other side of the package.
Lower tack PSAs that function like cohesive materials generate other concerns. Lower tack adhesive can be difficult to adhere to a package surface due to the low tack properties. Thus, fasteners created with lower tack PSAs may result in delamination of the PSA from the package surface upon opening of the package.